The semiconductor industry continues to develop lithographic technologies which can print ever smaller integrated circuit dimensions. The current state-of-the-art light sources for this industry are 248 nm and 193 nm excimer lasers.
The demands of the integrated circuit industry will soon exceed the resolution capabilities of 193 nm exposure sources, thus creating a need for a reliable exposure source at a wavelength significantly shorter than 193 nm. F2 excimer lasers operating at 157 nm are available and this source may be utilized to further reduce circuit dimensions.
The present state of the art in high energy ultraviolet light sources utilizes plasmas produced by bombarding various target materials with laser beams, electrons or other particles. Solid targets have been used, but the debris created by ablation of the solid target has detrimental effects on various components of a system intended for production line operation. A proposed solution to the debris problem is to use a frozen liquid or frozen gas target so that the debris will not plate out onto the optical equipment. However, none of these systems have proven to be practical for production line operation.
It has been well known for many years that x-rays and high energy ultraviolet radiation could be produced in a plasma pinch operation. In a plasma pinch an electric current is passed through a plasma in one of several possible configuration such that the magnetic field created by the flowing electric current accelerates the electrons and ions in the plasma into a tiny volume with sufficient energy to cause substantial stripping of outer electrons from the ions and a consequent production of x-rays and high energy ultraviolet radiation.
Typical prior art plasma pinch devices can generate large amounts of radiation suitable for proximity x-ray lithography, but are limited in repetition rate due to large per pulse electrical energy requirements, and short lived internal components. The stored electrical energy requirements for these systems range from 1 kJ to 100 kJ. The repetition rates typically did not exceed a few pulses per second.
What is needed is a production line reliable, simple system for producing high energy ultraviolet and x-radiation which operates at high repetition rates and avoids prior art problems associated with debris formation.
The present invention provides a high energy photon source. A pair of plasma pinch electrodes are located in a vacuum chamber. The chamber contains a working gas which includes a noble buffer gas and an active gas chosen to provide a desired spectral line. A pulse power source provides electrical pulses at repetition rates of 1000 Hz or greater and at voltages high enough to create electrical discharges between the electrodes to produce very high temperature, high density plasma pinches in the working gas providing radiation at the spectral line of the source or active gas. A fourth generation unit is described which produces 20 mJ, 13.5 nm pulses into 2 xcfx80 steradians at repetition rates of 2000 Hz with xenon as the active gas. This unit includes a pulse power system having a resonant charger charging a charging capacitor bank, and a magnetic compression circuit comprising a pulse transformer for generating the high voltage electrical pulses at repetition rates of 2000 Hz or greater. Gas flows in the vacuum chamber are controlled to assure desired concentration of active gas in the discharge region and to minimize active gas concentration in the beam path downstream of the pinch region. In a preferred embodiment, active gas is injected downstream of the pinch region and exhausted axially through the center of the anode. In another preferred embodiment a laser beam generates metal vapor at a location close to but downstream of the pinch region and the vapor is exhausted axially through the anode.
A reflection radiation collector-director collects radiation produced in the plasma pinch and directs the radiation in a desired direction. Embodiments are described for producing a focused beam. In one preferred embodiment a tandem ellipsoidal mirror is used to produce a focused beam having improved beam properties.
Preferred embodiments include a debris collector located between the pinch region and the radiation collector-director. The debris collector is comprised of a large number of passages each passage aligned with rays eminating from the pinch region which permits the passage of light rays which travel in straight lines but retards the passage of debris which travel along more random paths. In addition a gas flow is provided through the debris collector in a direction toward the pinch region to further retard debris penetration and active gas penetration toward the radiation collector-director.
In still another embodiment, the active gas is injected axially through the anode and exhausted along with buffer gas upstream of the debris collector.